Tonga's King George Tupou V, who relinquished absolute power to bring democracy to the South Pacific nation, has died at the age of 63, the country's prime minister announced Monday.

In an address to the nation Lord Tu'ivakano said the monarch, renowned for his eccentricities as much as his democratic legacy, died on Sunday.

Government officials said Tu'ivakano announced the death in Tongan on public radio but they declined to provide further details until an English translation had been officially approved.

New Zealand Prime Minister John Key said the monarch, who came to the throne following the death of his father King Taufa’ahau Tupou IV in 2006, died in hospital in Hong Kong on Sunday night.

Key said he was saddened by the king's death and paid tribute to the reforms he introduced in the impoverished island nation of 115,000.

"He believed that the monarchy was an instrument of change and can truly be seen as the architect of evolving democracy in Tonga," the New Zealand leader said. "This will be his enduring legacy."

King Tupou V's cause of death was not immediately known but he underwent surgery to have a kidney removed in Los Angeles last year after a cancerous tumour was discovered.

Before ascending to the throne, the king was best known for his globe-trotting lifestyle and love of elaborate uniforms, including colonial-era pith helmets and jackets with elaborate gold braiding.

He often wore a monocle and was driven around the capital Nuku'alofa in a black London taxi, sometimes attracting criticism for being out of touch with his people.

But despite his eccentricities, which also included sailing model boats in his swimming pool and playing computer games, the rugby-mad monarch introduced significant reform to his kingdom.

Key said the king "was a long-time champion of a more open system of government for Tonga", who saw the monarch as integral to Tongan's culture but wanted a more representative parliament.

He was sworn in as king of one of the world's last absolute monarchies in September 2006, with the need for reform taking on added urgency when riots two months later left eight people dead and much of downtown Nuku'alofa destroyed.

When Tupou V was finally crowned in an ancient Tongan ceremony, more than 200 nobles and chiefs presented dozens of slaughtered pigs and hundreds of baskets of food in tribute.

He was offered a bowl of kava, a mild narcotic drink made from plant roots, to signify his sovereignty over Tonga, a country spread over 170 islands.

Within two years, the Oxford University-educated monarch had made good on his pledge for democracy and the people of Tonga voted in their first popularly elected parliament in 2010 ending 165 years of feudal rule.

The democratic changes replaced a system where the public could only vote for nine of the 33 representatives in a parliament dominated by a clique of nobles selected by the king, who also chose the prime minister and cabinet.

One of his last public appearances was an audience with Pope Benedict XVI in the Vatican last month.